A common application for a light dimmer is a two wire connection. A block diagram of a typical two wire connection is shown in FIG. 1. In this particular case, dimmer's switching element 15 is connected between line terminal 11 and a load 16. The neutral terminal 12 is connected to the other side of the load. The dimmer's power supply 18 is connected across the switching element terminals. Design of this type of power supply represents a challenge as utilization of the available AC power source depends on the status of the load. AC input can be utilized only when the dimmer's switching element is OFF; this happens only during a period of time around the zero crossing. This time is minimal when the dimmer is set for maximum brightness. FIG. 2 represents an example of a dimmer AC waveform 20 where a triac is utilized as a switching element. Time periods 22 (Tl to T2, etc.) represent the time that the load is ON. Time periods 21 (T0 to T1, T2 to T3, etc.) represent the time that the load is OFF, during which AC line power is available for the dimmer's power supply. This limits the amount of power available for power supply 18. A similar problem may arise in home energy management systems, in which the power switching device is placed in series with a 24V or 120V power source and a load (as discussed in U.S. Pat. No. 4,678,985).
Conventional power supplies for two line dimmers are often based on a linear analog approach, including different capacitor charging schemes with a linear post regulation. Examples of power supplies utilizing this approach are shown in U.S. Pat. Nos. 4,334,184; 4,504,778; 4,678,985; and 5,600,552. These designs have a number of drawbacks, including need for bulky components and inefficiency, as they continuously dissipate energy and draw current while the device is in its OFF state. The implementation cost of linear power supplies may also be high.
A switching power supply can avoid the above-noted problems. A switch mode power supply regulates by connecting high input voltage to the low voltage power supply circuit for a short period of time. In a common approach, when a power supply switching element (such as a transistor) turns on, energy is delivered to an inductor, and in some cases to the output capacitor and a load. When the transistor turns off, the stored energy in the inductor is delivered to an outnut filter capacitor and a load. When the transistor conducts, it operates at fill current with minimum voltage drop, which results in little wasted power.
The minimum operating voltage of the power supply switching device represents a design restriction which becomes critical for two wire lighting applications. As shown in FIG. 2, since the AC input power can be utilized only during short intervals of time while the load is OFF, peak voltage levels which develop during this time may not be high enough for the power supply to operate properly. Any increase in the power supply input voltage level will significantly improve its performance. In addition, implementation of a dimmer's switching power supplies can be further restricted by a common requirement for an unobstructed access to the line terminal.
The present invention offers a solution that overcomes both of these problems.